Initializing the WebUSB device detector module causes the USB service to
be initialized and enumerate all USB device connected to the system.
This can cause unnecessary delay during browser startup. The purpose of
this module is to detect devices connected after Chrome starts to
display a notification so it doesn't matter if it is initialized after
first page load.

An UMA histogram for the time spent initializing the detector is also
added to aid further investigation of this issue.

BUG=642107

Review-Url: https://codereview.chromium.org/2295023002
Cr-Commit-Position: refs/heads/master@{#415971}